1. Field of the Invention
The present invention relates generally to automobile fuel injection systems, and more particularly to a fuel injection control apparatus of an internal combustion engine that corrects fuel injection volume based on inferred fuel temperature.
2. Description of Related Art
If an internal combustion engine operates continuously under a heavy load for a long period of time, the temperature of the engine increases to a high value, and fuel evaporative emission, referred to hereafter as vapor, is produced in engine fuel pipes. If such vapor is generated, the fuel injection volume becomes smaller than a demanded value, resulting in a shift of the air-fuel ratio to the lean region. As is disclosed in Japanese Patent Laid-open No. Sho. 56-81230, to solve this problem, fuel temperature may be detected by using a fuel temperature sensor and, as the fuel temperature increases to a high value, the fuel injection volume may be corrected by increasing the volume. Alternatively, when the temperature of cooling water increases to a high value, the fuel pressure may be raised to correct the fuel injection volume by increasing the volume, as disclosed in Japanese Laid-open Application No. Hei. 5-125984.
However, a shift of the air-fuel ratio accompanying an increase in fuel temperature is not only attributed to a change in vapor generation volume, but also to a change in fuel density (the fuel itself, excluding vapor) attributed to fuel temperature. That is, a change in injected fuel temperature results in a change in injected fuel weight even if the volume of injected fuel remains the same. As a result, if the temperature of injected fuel changes, the air-fuel ratio is shifted.
To cope with a change in fuel density due to such a change in fuel temperature, the temperature of fuel may be detected by using a fuel temperature sensor and the fuel injection volume is corrected in accordance with a change in fuel density which is caused by a change in fuel temperature, as is disclosed in Japanese Patent Laid-open No. Sho. 52-133419.
As described above, a shift of the air-fuel ratio accompanying a change in fuel temperature is caused by two factors, namely, a change in generated vapor and a change in fuel density. In each of the solutions according to the conventional techniques described above, only one factor causing the shift is taken into consideration, making it impossible to correct a shift of the air-fuel ratio accompanying a change in fuel temperature, that is, a shift of the fuel injection volume, with a high degree of precision. In addition, when a fuel temperature sensor is required to detect the fuel temperature, increased system cost results.